The invention relates generally to sound reproduction. More specifically, the invention relates to multiple channel sound reproduction systems having improved listener perceived characteristics.
Multiple channel sound reproduction systems which include a surround-sound channel (often referred to in the past as an "ambience" or "special-effects" channel) in addition to left and right (and optimally, center) sound channels are now relatively common in motion picture theaters and are becoming more and more common in the homes of consumers. A driving force behind the proliferation of such systems in consumers' homes is the widespread availability of surround-sound home video software, mainly surround-sound motion pictures (movies) made for theatrical release and subsequently transferred to home video formats (e.g., videocassettes and videodiscs).
Although home video software formats have two-track stereophonic soundtracks, those two tracks carry, by means of amplitude and phase matrix encoding, four channels of sound information--left, center, right, and surround, usually identical to the two-track stereophonic motion-picture soundtracks from which the home video soundtracks are derived. As is also done in the motion picture theater, the left, center, right, and surround channels are decoded and recovered by consumers with a matrix decoder, usually referred to as a "surround-sound" decoder. In the home environment, the decoder is usually incorporated in or is an accessory to a videocassette player, videodisc player, or television set/video monitor. Although nearly universal in motion picture theater environments, the center channel playback is often omitted in home systems. A phantom-image center channel is then fed to left and right loudspeakers to make up for the lack of a center channel speaker.
Motion picture theaters equipped for surround sound typically have at least three sets of loudspeakers, located appropriately for reproduction of the left, center, and right channels, at the front of the theater auditorium, behind the screen. The surround channel is usually applied to a multiplicity of speakers located other than at the front of the theater auditorium.
It is the recommended and common practice in the industry to align the sound system of large auditoriums, particularly a motion picture theater's loudspeaker-room response, to a standardized frequency response curve or "house curve". The current standardized house curve for movie theaters is a recommendation of the International Standards Organization designated as curve X of ISO 2969-1977(E). The use of a standardized response curve is significant because in the final steps of creating motion picture soundtracks, the soundtracks are almost always monitored in large (theater-sized) auditoriums ("mixing" and "dubbing" theaters) whose loudspeaker-room responses have been aligned to the standardized response curve. This is done, of course, with the expectation that such motion picture films will be played in large (theater-sized) auditoriums that have been aligned to the same standardized response curve. Consequently, motion picture soundtracks inherently carry a built-in equalization that takes into account or compensates for playback in large (theater-sized) auditoriums whose loudspeaker-room responses are aligned to the standardized curve.
The current standardized curve, curve X of ISO 2969, is a curve having a significant high-frequency rolloff. The curve is the result of subjective listening tests conducted in large (theater-sized) auditoriums. A basic rationale for such a curve is given by Robert B. Schulein in his article "In Situ Measurement and Equalization of Sound Reproduction Systems", J. Audio Eng. Soc., April 1975, Vol. 23, No. 3, pp. 178-186. Schulein explains that the requirement for high-frequency rolloff is apparently due to the free field (i.e., direct) to diffuse (i.e., reflected or reverberant) sound field diffraction effects of the human head and ears. A distant loudspeaker in a large listening room is perceived by listeners as having greater high frequency output than a closer loudspeaker, if aligned to measure the same response. This appears to be a result of the substantial diffuse field to free field ratio generated by the distant loudspeaker; a loudspeaker close to a listener generates such a small diffuse to direct sound ratio as to be insignificant.
More recently the rationale has been carried further by Gunther Theile ("On the Standardization of the Frequency Response of High-Quality Studio Headphones", J. Audio Eng. Soc., December 1986, Vol. 34, No. 12, pp. 956-969) who hypothesized that perceptions of loudness and tone color (timbre) are not completely determined by sound pressure and spectrum in the auditory canal. Theile relates this hypothesis to the "source location effect" or "sound level loudness divergence" ("SLD") which occurs whenever auditory events with differing locations are compared: a nearer loudspeaker requires more sound level (sound pressure) at the ear drums to cause the same perceived sound loudness as a more distant loudspeaker and the effect is frequency dependent.
It has also been recognized that the sound pressure level in a free (direct) field exceeds that in a diffuse field for equal loudness. A standard equalization, currently embodied in ISO 454-1975 (E) of the International Standards Organization, is intended to compensate for the differences in perceived loudness and, by extension, timbre due to frequency response changes between such sound fields.
Perceived sound loudness and timbre thus depends not only on the location at which sound fields are generated with respect to the listener but also on the relative diffuse (reflected or reverberant) field component to free (direct) field component ratio of the sound field at the listener.
One major difference between the home listening environment and the motion picture theater listening environment is in the relative sizes of the listening rooms--the typical home listening room, of course, being much smaller. While there is no established standard curve to which home sound systems are aligned, the high-frequency rolloff house curve applicable to large auditoriums is not applicable to the considerably smaller home listening room because of the above-mentioned effects.
Unlike home video software media having soundtracks transferred from motion picture film soundtracks, recorded consumer software sound media (e.g., vinyl phonograph records, cassette tapes, compact discs, etc.) have a built-in equalization that compensates for typical home listening room environments. This is because during their preparation such recordings are monitored in relatively small (home listening room sized) monitoring studios using loudspeakers which are the same or similar to those typically used in homes. Relative to large auditorium theater environments, the response of a typical modern home listening room-loudspeaker system or a small studio listening room-loudspeaker system can be characterized as substantially "flat", particularly in the high-frequency region in which rolloff is applied in the large auditorium house curve. A consequence of these differences is that motion pictures transferred to home video software media have too much high-frequency sound when reproduced by a home system. Consequently, the musical portions of motion picture soundtracks played on home systems tend to sound "bright". In addition, other undesirable results occur--"Foley" sound effects, such as the rustling of clothing, etc., which tend to have substantial high-frequency content, are over-emphasized. Also, the increased high-frequency sensitivity of home systems often reveals details in the makeup of the soundtrack that are not intended to be heard by listeners; for example, changes in soundtrack noise level as dialogue tracks are cut in and out. These same problems, of course, occur when a motion picture soundtrack is played back in any small listening environment having consumer-type loudspeakers, such as small monitoring studios.
There is yet another difference between the home sound systems and motion picture theater sound systems that detracts from creating a theater-like experience in the home. It has been the practice at least in certain high-quality theater sound systems to employ loudspeakers that provide a substantially directional sound field for the left, center, and right channels and to employ loudspeakers that provide a substantially non-directional sound field for the surround channel. Such an arrangement enhances the perception of sound localization as a result of the directional front loudspeakers while at the same time enhancing the perception of ambience and envelopment as a result of the non-directional surround loudspeakers.
In contrast, home systems typically employ main channel (left and right channel) loudspeakers designed to generate a compromise sound field that is neither extremely directional nor extremely non-directional. Surround channel loudspeakers in the home are usually down-sized versions of the main channel loudspeakers and generate sound fields similar to those of the main channel loudspeakers. In the home environment, little or no attention has been given to the proper selection of directional characteristics for the main channel and surround channel speakers.
Also, in both home and theater systems, including the above-mentioned high-quality theater sound systems, no compensation has been employed for the differences in listener perceived timbre between the main channels and the surround channel. For example, sounds which move from the main channels to the surround channel or vice-versa (sounds "panned" off or onto the viewing screen) undergo timbral shifts. Such shifts in timbre can be so severe as to harm the ability of the listener to believe that the sound is coming from the same sound source as the sound is panned.
The inventor has discovered that the aforementioned equalization standard, currently embodied in ISO 454-1975 (E) of the International Standards Organization, cannot be used as a basis to properly compensate for the listener perceived timbre differences between the main and surround channels.
The inventor believes that there are two main causes for the listener perceived timbral shift between the main and surround channels. The first is timbre changes due to comb filtering. Comb filtering may arise from the operation of multiple surround loudspeakers or from deliberately added electronic comb filters used to simulate a surround array with only two loudspeakers. The second cause is frequency response differences due to the human head related transfer function. In addition, the difference in character between the direct sound field generated by the main channel loudspeakers and the diffuse sound field generated by the surround channel loudspeakers may be an additional factor.
In addition, with respect to home systems and to the above-mentioned high quality theater sound systems, a single (monophonic) surround-sound channel is applied to multiple loudspeakers (usually two, in the case of the home, located to the left and right at the sides or rear of a home listening room and usually more than two, in the case of a motion-picture theater, located on the side and rear walls). The result of driving the two sides of the head with the same signal is that the surround-sound channel sounds to a listener seated on the center line as though it were in the middle of the head.